The post-Watergate crusade against corruption has spurred aggressive prosecutors across America to enforce the law with a vengeance. It is the Day of the Prosecutor.

Some don't always wait for a crime to occur. They organize a task force, select the "bad guys" and crack down. They sometimes conduct what one prosecutor described approvingly as "investigation by terrorism." Suspects are harassed; witnesses are intimidated; the uncooperative are jailed for contempt.

Some prosecuting attorneys use grand juries to hound victims and compel their associates to testify against them. Witnesses can be subpoenaed and interrogated under oath. Those who refuse to answer questions can be cited for contempt; those who lie can be indicted for perjury.

Unfortunately, this has perverted the original purpose of the grand jury, which was established to protect innocent citizens from overzealous prosecutors. The grand jury was supposed to consider the prosecutor's charges in secret, weed out the irresponsible accusations and permit only substantive charges to reach open court.

Now the grand jury, like the appendix, has lost its function and no longer filters out the poison. Raw, unsubstantiated allegations not only reach open court but the open press. The derogatory information is sometimes leaked by the prosecutors.

Martin G. Holleran, former executive director of the New Jersey State Commission of Investigations, told prosecutors at a closed-door seminar that every Justice Department task force should have "a special grand jury impaneled exclusively" for its use. "You can use this grand jury as an investigative and intelligence-gathering tool by calling organized crime figures" before it. This tactic, he acknowledged, "is subject to cries of harassment. . . . But so be it; that is the way it goes."

Many lawmen were elated over the 1974 Supreme Court ruling that allows evidence seized in violation of the Constitution to be used in grand-jury proceedings. As Maryland Judge Charles E. Moylan Jr. observed ruefully: "The prosecutor can burn the Bill of Rights seven days out of seven an bring the fruits of unconstitutional activity to a grand jury. No court in the country has the power to ook behind what the grand jury considers or why it acts as it does."

Not only are aggressive prosecutors using grand juries to harass alleged wrongdoers, with little real regard for constitutional rights and guarantees; they are also sending suspected criminals to prison without a trial.

Martin Holleran explained to his fellow prosecutors how to do it. Simply grant the suspect immunity so he can't plead the Fifth Amendment against self-incrimination, Holleran advised. Organized crime figures, he said, have "a code fo silence." Therefore, they usually "refuse even to give their addresses" and place themselves in contempt. "They have the key to the jailhouse door in their own pocket," he said.

No doubt many of these witnesses belong behind bars. But if they can be put in prison without a trial, every citizen suddenly is in jeopardy. As one troubled prosecutor put it: "That's totalitarian. It offends me."

Rodney Sager, a former Assistant U.S. Attorney for Virginia, warned a House Judiciary subcommittee behind closed doors: "The simply fact is today our system clothes the prosecutor with virtually unbridled powers. It's interesting to note from the historic comments that the so-called 'function of the grand jury is to act as a protector of the individual's rights. I would respectfully suggest that in modern-day practice that term is completely meaningless. To the contrary, the person protected by the modern-day grand jury is the prosecutor."

Citing his own experience as a prosecutor, Sager testified that grand jurors "yield almost completely to the will, advice and actions of the prosecutor." He cautioned that "the young prosecutor who has little past experience finds himself with awesome power, and it is very simple for even a well-meaning prosecutor to take advantage of such a situation."

Again drawing on his own experience, Sager said: "I observed situations where prosecutors, off the record, told grand jurors that certain witnesses were crooks, that they were con men, that they were expected to be evasive in their responses to questions and not to pay attention to anything that they might have to say.

"I have been in the grand-jury rooms when the prosecutor, off the record, joked with the grand jurors about the testimony of the particular witness. . . . You get the opportunity to get a witness alone without his counsel. And you can rant and rave to your heart's content with that witness without anyone, really, ever having any idea about what's going on inside the grand-jury room."

Explained Sager: "The prosecutor has full authority to tell the grand-jury stenographer what to take down and what not to take down. And, my friends, you'd better believe that if the prosecutor has some strong words to say about a witness, he advises the court reporter to cease the transcription, so those words are not a part of any record."

The former prosecutor summarized his own testimony in these blunt words: "It's ironic that the grand jury, which was meant to protect against an overly aggressive prosecutor, is a system that has, in fact, brought into being the overly aggressive prosecutor."